I don't think "sponsored results" are bad per se. However, I don't see a reason to use the Gnutella infrastructure for this. I suggest you implement this as a opt-in feature and use a few dedicated servers for this. You could use some kind of QRP to ensure that those servers see only results that would match - to reduce the load on those and to prevent spying on users.

You could even use a cluster of ultrapeers and handle this in a similar way as "locale preferences" are used by LimeWire. I recommend to refrain to utilize the common Gnutella network for such advertising. Spam is a fuzzy term but many people would certainly regard it as such. I definitely consider Google Adsense as spam - in many cases. You as LimeWire should know very well how much damage this (along with domain squatting) causes to you and your reputation.